1. Field of the Invention
The present invention relates to a process for molding and vulcanizing rubber products. More particularly, it relates to a process for molding and vulcanizing rubber products by using an improved curing bladder. The process of this invention is particularly suitable for molding and vulcanizing tires.
2. Description of the Prior Art
It has been a conventional practice for the production of pneumatic rubber tires for vehicles to mold and cure a green tire using a molding press. A green tire placed in a molding press is pressed against the mold surface by a bladder which is expanded by a fluid introduced therein. In this molding process, a green tire is molded and vulcanized in conformity with the mold surface that determines the tread pattern and the side wall structure.
The bladder used for molding and vulcanizing rubber products such as tires as mentioned above has been made of an organic rubber, particularly butyl rubber. A bladder of organic rubber has a disadvantage in that it is not readily released from the inside of a tire and it is bent when a tire is demolded. This causes defective molding of tires. In addition, the surface of a bladder of organic rubber becomes rough due to wear and this rough surface easily sticks to the inside of a tire in the curing cycle and does not separate from the inside of a tire after curing when the bladder is shrunk. Moreover, the rough surface entraps air bubbles between the bladder and the inside of a tire being vulcanized, and the entrapped air bubbles prevent the smooth heat transfer and cause defective vulcanization.
For the reasons mentioned above, the conventional curing bladder needs a release agent of silicone emulsion or the like that improves the lubricity between the bladder and the inside of the tire. The application of a release agent to the inside of the tire, however, is not desirable because it requires an additional production step and adversely affects the vulcanization step, resulting in defective products. In addition, the release agent adds to the items of inventory.
In order to eliminate the above-mentioned problems, there has been proposed a method for modifying the surface of the organic rubber bladder with silicone. The modification with methylhydrogensilane or dimethylhydrogensilane is disclosed in Japanese Patent Application Laid-open No. 111394/1982, and the modification with hydroxylsilane is disclosed in Japanese Patent Application Laid-open Nos. 111393/1982 and 119992/1982.
The conventional surface modification with silicone as mentioned above improves the releasability of the bladder surface from the tire inside, obviating the use of a release agent in the molding and vulcanization of tires. On the other hand, the surface modification impairs the durability of a bladder to such an extent that it is not practical. Under these conditions, there has been a strong demand for a practically useful curing bladder which has outstanding physical properties such as mechanical strength, heat resistance, and hot water resistance, as well as good releasability.